1. Field of Invention
This invention relates to a method of fabricating bias field magnets for magnetic bubble memory devices as well as to the magnets produced thereby. More specifically, the present invention is directed toward such a method to produce bias field magnets having a more uniform magnetic field as well as being of a simplier construction.
2. Description of the Prior Art
A particular bias field magnet for magnetic bubble devices as well as the mechanism and structure of such devices themselves is provided in Bobeck, Boneyhard and Ge, Magnetic Bubbles--An Emerging New Memory Technology, Proceedings of IEEE, Volume 63, Number 8, Aug. 1975, Pages 1176-1195. The bias field described therein is provided by a modified Watson-type magnet consisting of a pair of soft ferrite plates and Index magnet inserts. In this structure, the soft ferrite sheets improve the bias field uniformity of the hard ferrite magnets. However, the bias field uniformity can still be improved by insuring that there are no air gaps between the hard and soft ferrite magnets. Furthermore, it is proposed that a structure can be provided which can be fabricated as a one piece assembly thereby reducing the cost of manufacture. Another example of the modified Watson-type magnet is disclosed in the Chow, et al. U.S. Pat. No. 5,927,397.
Magnetic bubble memory devices employ the mechanism of cylindrical magnetic domains formed within a thin magnetic layer which may be a magnetic garnet grown epitaxially on a non-magnetic garnet substrate. The magnetic domains are moved within the layer along propagation paths formed therein under the influence of external magnetic fields. Movement of the magnetic domains or bubbles is caused by creating a rotating magnetic field within the plane of the magnetic layer. In addition, a bias field is applied normal to that plane to provide a stable condition for magnetic domains at approximately their mid range diameter between strip out and collapse.
The bias field requirement can vary from garnet wafer to garnet wafer over a wide range and, therefore, it is required that the bias field be as uniform as possible to allow greater margin in the circuit and wafer designs and the processes for creating the wafers. It is preferable to have a bias field uniform within half a percent of its nominal value normal to the plane of the garnet wafer in the volume of the epitaxial bubble film. The above described Bobeck, et al article and the Chow et al Patent describe structures are designed to achieve more uniform fields. However, the bias field magnets of those disclosures are formed of separate hard and soft magnetic bodies which later must be bonded together. This results in air gaps between the respective magnetic layers which tend to disturb the magnetic field uniformity. In addition, in fabrication, the surfaces must be ground in order to obtain a high degree of flatness and this contributes to the costs of the fabrication.
It is, then, an object of the present invention to provide an improved bias field magnet for magnetic bubble memory devices which magnet has a highly uniform magnetic field.
It is another object of the present invention to provide a bias field magnet having a highly uniform field, the uniformity which is not disturbed by air gaps in the structures of the magnet.
It is still a further object of the present invention to provide a bias field magnet which has a highly uniform field and which also is simple to fabricate.